Paperwebs are generally processed in a calender stack to establish the final caliper of the web and to improve the smoothness and uniformity of the web as well as other properties. In many instances, the calender stack also includes a waterbox. As the paper web is pressed through the calender stack, the waterbox is used to apply a controlled amount of water to the web, often together with certain additives such as starch.
The waterbox is a long, somewhat elongate trough-like structure with a lip extending from and along one side adjacent a roll on the calender stack which is used to supply the water-starch or other solution or mixture to the web as it passes through the adjacent calender nip. The waterbox is typically opened from overhead and liquid solution is conventionally supplied to the waterbox via a plurality of overhead nozzles spaced-apart along the length of the box. The nozzles are arranged so their openings cause a starch mixture to be delivered into the open top of the waterbox at spaced-apart locations from above the mixture level.
One problem with this overhead nozzle design is that the nozzles tend to become encrusted with dried starch residue because of splashing, which is inherent in the operation of the waterbox and calender. Due to the elevated temperatures of the calender stack, the water quickly evaporates from the splashings, causing a rapid build up of starch on the nozzles and other structure in and around the waterbox. Eventually, pieces of this dried starch break loose and enter the calender nip creating defects in the paper. Such paper defects are sometimes referred to as paper “scabs”.
In order to limit this starch buildup from fouling the calender stack and causing paper scabbing, it has been necessary to make frequent stoppages of the calender and remove the starch buildup using high pressure and/or high temperature cleaning sprayers. Because of the high pressures and temperatures, such cleanup procedures pose a significant threat of heat stress or other injury to workers. Moreover, valuable production time is lost with each stoppage of the papermaking line.
In addition to scabbing problems, conventional waterboxes also tend to develop non-uniformities in the concentration of aqueous mixture components such as starch, resulting in more highly concentrated zones of starch within the waterbox which often appear between overhead nozzles. These zones are sometimes called “hot spots”. The formation of hot spots can lead to wrinkling and/or corrugating of the paperweb, due to sharp or abrupt concentration differentials in the starch applied to the web.
Accordingly, it is an object of the invention to provide a novel waterbox design which improves upon conventional designs in regard to scabbing, hot spots, and other problems.